The present invention relates generally to power amplifier systems and mobile communications terminal devices, and more particularly to architectures suitably adaptable for use with high frequency power amplifier systems for over-the-air radiocommunications services using Schottky barrier gate metal semiconductor field effect transistors (MESFETs) made of compound semiconductor and also mobile communications terminal devices using the same.
In portable or handheld mobile communications terminal devices such as personal digital cellular (PDC) or personal handyphone systems (PHS) or the like, radiocommunications are performed by use of carrier waves in microwave bands at frequencies of 1 gigahertz (GHz) or higher. Due to this, power amplifier circuitry for transmit signals and pre-amplifier circuitry for receive signals are typically designed to employ gallium arsenide (GaAs) MESFETs that operate at higher speeds than standard silicon transistors.
General teachings about mobile communications terminal devices are found in several printed publications including, for example, xe2x80x9cNIKKEI ELECTRONICS,xe2x80x9d by Nikkei.BP Corp., Apr. 16, 1990 (No. 497), p. 121.
While this mobile communications terminal device requires relatively large electrical power of approximately 1 watt (w) for over-the-air signal transmission, it is also required that the device be smaller in size and longer in operation time period on a battery in order to increase the portability of such mobile communications terminal. In view of this, use of battery-based single power supply drive scheme is preferable, which in turn requires low power consumption in a viewpoint of guarantee of long term operabilities.
Incidentally, in cases where GaAs MESFETS are utilized at high frequency bands, n-channel type MESFETS are generally employed in order to take full advantage of inherent significance of electron mobility therein. Accordingly, the following description will be devoted to the case of n-channel MESFETs, except as otherwise stated to the contrary.
In addition, in prior art MESFETs, those of the depression type that are relatively deep in threshold voltage (e.g. Vth=xe2x88x921V, or more or less) are used in order to gain a significant amplification degree.
In case the MESFET of relatively deep Vth is used with its source grounded, it should be required that a gate bias of negative potential be applied thereto, which in turn requires separate use of a negative power supply voltage in addition to a positive power supply voltage. The amplifier system requiring such power supplies of both the positive and negative polarities is incapable of being driven by a single power supplyxe2x80x94when an attempt is made to forcibly drive the system by using a single or unitary power supply, a specific scheme will be required for employing a DC-DC converter to generate from a positive power supply a negative voltage for use as the negative power supply.
Unfortunately the DC-DC converter employment scheme does not come without accompanying penalties as to an increase in power consumption and also an increase in parts mount area, which will become contradictory to the need for small size and long term battery drivability posed on mobile communications terminals.
Then, a need arises to consider employment of special circuitry for applying a gate bias voltage of zero volts or of positive polarity, which circuitry is typically designed to make use of certain GaAs MESFETs of either relatively shallow depression type or enhancement type with vth being positive in polarity, as amplifying elements for use in a power amplifier circuit of mobile communications terminals.
In view of the fact that a GaAs MESFET constitutes a Schottky junction FETxe2x80x94in other words, a gate and source make up a Schottky diodexe2x80x94while an n-channel MESFET is used with its source grounded, application of a positive voltage to the gate would result in creation of a forward voltage with respect to the Schottky diode. This in turn makes it necessary that a positive voltage capable of application to the gate must be less than or equal to a specified voltage (Vf) at which a gate current (forward current) behaves to rapidly increase. This requirement comes because even upon applying a gate voltage of Vf or higher, a depletion layer underlying a gate electrode has already disappeared leading to an inability to control a drain current which can result in saturation of the drain current. On the other hand, a minimal value of the gate voltage capable of being applied in the negative direction becomes near or around the Vth value. This can occur because even when applying a gate voltage of less than or equal to Vth, a channel region has already been cut off by a depletion layer so that any drain current is no longer flowing therein.
In brief, while a linear region which permits the drain current to vary with a change in gate voltage is needed in order to take out the drain current of a MESFET as the intended amplified signal, the use of this region means that the gate voltage must fall within a limited range of from Vth to Vf.
Accordingly, when compared to a deep vth depression type MESFET, MESFETs of the shallow Vth depression type or positive vth enhancement type become narrower in range insuring gate voltage applicability. Generally the drain current gets larger when applying the gate voltage maximally; thus, the drain current tends to increase in amplitude in a way proportional to the amplitude of such gate voltage. Due to this, in the case of the MESFETs of relatively shallow depression type or enhancement type, it will possibly happen that any sufficient drain current is hardly obtainable. This would result in that the intended output or gain of the amplifier system is by no means attainable during a high frequency operation thereof, which leads to occurrence of a serious bar to the quest for higher performance in mobile communications terminals.
On the other hand, as has been recited in Japanese printed matter such as for example xe2x80x9cCOMPOUND SEMICONDUCTORS,xe2x80x9d Nikkan Kougyou Shinbun-Sha (this means in English xe2x80x9cDaily Engineering Newspaper Corp.xe2x80x9d), Jan. 30, 1986 at p. 164, the current density, J, of a forward current flowing between a metal and a semiconductor that are in Schottky junction is given as:
J=A*T2exp(xe2x88x92qxcfx86B/kT)(exp(qV/nkT)xe2x88x921),
where xe2x80x9cA*xe2x80x9d is the effective Richardson constant, T is the absolute temperature (K), q is the elementary charge carrier, xcfx86B is the Schottky barrier (V), k is the Boltzmann""s constant, V is the applied voltage (V), and n is the ideal parameter or coefficient, which is expected to fall within a range of 1.0 to 1.3 when the Schottky junction is superior.
Assuming that exp (qV/nkT) is established and xe2x80x9cnxe2x80x9d is nearly equal to 1, the current density J behaves to increase exponentially at or near a point whereat V goes beyond xcfx86B as readily appreciated by those skilled in the art to which the invention pertains. Such situation is equivalent to the phenomenon that a gate current rapidly increases with an increase in gate voltage in source-grounded MESFETs. In short, Vf is strongly related to xcfx86Bxe2x80x94the greater xcfx86B, the larger Vf. Accordingly, it may be considered that the use of those materials with large values of xcfx86B for the gate electrode is effective in order to increase the Vf value to thereby likewise increase the range of application of the gate voltage exhibiting amplification functionality.
Regrettably it has been known among experts in the art that even when a metal with Schottky junctionability is formed on the surface of GaAs, xcfx86B does not vary in accordance with the kind of a metal, that is, the work function of such metal, and thus xcfx86B remains almost constant. It is considered that this is owing to greatness of the surface energy level density on GaAs surfaces or alternatively pinning effects occurring due to creation of an intermediate layer.
In prior art n-channel GaAs MESFETs employed in many cases, the gate electrode is typically made of a tungsten silicide (WSi)-based material, wherein even if this gate electrode is modified to replace it with either aluminum (Al) or molybdenum (Mo) by way of example, the resultant Schottky barrier xcfx86B will never change significantly due to the presence of pinning effects. For this reason, it remains difficult to increase the gate voltage applicable range by increasing Vf, which in turn makes it difficult for those MESFETs of the relatively shallow-depression type or enhancement type to provide a sufficient drain current for improvement in output or gain of the amplifier system during a high frequency operation thereof to thereby increase the performance of mobile communications terminals.
Additionally in cases where n-channel GaAs MESFETs of the relatively shallow depression type or enhancement type are employed with the source grounded, the resulting range of a gate voltage becomes narrower, which causes the stability of such gate bias voltage to significantly affect the signal-to-noise ratio (SNR) concerned. In view of this, the stability of power supply voltage will especially become important.
It is therefore an object of the present invention to improve the gain of a high frequency power amplifier system that is inherently designed to be driven by a single power supply.
It is another object of this invention to reduce electrical power as consumed by the high frequency power amplifier system.
It is still another object of the invention to improve the stability of such high frequency power amplifier system.
It is a further object of the invention to increase an output of a mobile communications terminal device offering drivability on a single power supply with low power consumptionxe2x80x94namely capable of exhibiting long term operation on a batteryxe2x80x94while improving performance.
These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.
(1) A power amplifier system incorporating the principles of the present invention is a specific power amplifier system which employs as its amplifying element a Schottky barrier insulated gate field effect transistor also known as metal semiconductor field effect transistor (MESFET) with its source terminal coupled to the ground for receiving a drain bias voltage and a forward gate bias voltage of zero volts or low potentials as supplied from a unipolar power supply unit and for amplifying an input signal being superposed with the gate bias voltage to thereby generate and issue an output signal indicative of a change in drain current, wherein the MESFET is such that when a forward direct current (DC) gate voltage is applied to a gate terminal with a source terminal grounded, the DC gate voltage becomes greater than or equal in value to 0.65 volts (V), which voltage causes a gate current value per gate width of 100 micrometers (xcexcm) to go beyond 100 microamperes (xcexcA).
It has been stated that prior art MESFETs have a gate electrode typically made of tungsten silicides in many cases and that the Schottky barrier xcfx86B will not vary significantly even where such gate material is changed. As the methodology for evaluation of such Schottky barrier xcfx86B, the inventors as named herein have been introduced a specific concept for defining as the Vf value strongly relating to xcfx86B the value of a DC gate voltage which permits a gate current value per gate width of 100 xcexcm to exceed 100 xcexcA in cases where a forward DC gate voltage is applied to the gate terminal with a source terminal coupled to ground.
More specifically, as the Schottky junction is established between the gate and the source as grounded, applying a forward voltage (positive voltage in the case of an n-channel MESFET) to the gate terminal results in a forward current flowing in the gate terminal at the current density J. As previously discussed,
J=A*T2exp(xe2x88x92qxcfx86B/kT)(exp(qV/nkT)xe2x88x921),
and, under a condition that permits establishment of qV/nkT greater than 3, that is, upon application of a certain gate voltage V, we obtain
J=A*T2exp(xe2x88x92qB/kT)exp(qV/nkT) =xcex1xc2x7exp(xe2x88x92xcex2xcfx86B)exp(xcex2V/n),
where xcex1=A**T2, and xcex2=q/kT. Hence, calculating the natural logarithm of the above equation, we have
V=n+xcfx86B+(n/xcex2)1n(J/xcex1).
Here, let Vf be defined in the way stated above while assuming n=1, then
Vf=xcfx86B+(1/xcex2)1n(J0/xcex1),
where J0 is the gate current density equivalent to 100 xcexcA per gate width of 100 xcexcm. The inventors"" evaluation through experimentation has reveled the fact that the term (1/xcex2)1n(J0/xcex1) may be approximated as 0. Thus, we finally obtain Vf=xcfx86B.
Based on the definition above, the value Vf of prior art MESFETs adaptable for main use in prior art equipment, especially in mobile communications terminals, has been evaluated to demonstrate that Vf is 0.56V in case a gate electrode of tungsten silicide was formed on GaAs and that the value of Vf stays at 0.6V or less and hardly exceeds it.
In view of this fact, the present invention is for letting Vf in such definition be greater than or equal to 0.65V to thereby widen or expand the range of a gate voltage capable of being applied as an input signal, thus increasing the amplitude of a drain current that may be derived as an amplified output.
In other words, in accordance with the instant invention, it becomes possible to increase either an output or the gain of a power amplifier system employing a MESFET, thereby improving the performance of such system as a whole.
The ability to increase the output or gain of the power amplifier system in accordance with the invention will be set forth in detail with reference to FIGS. 1 to 3 below.
FIGS. 1-3 are graphs showing some characteristic curves for use in explaining an operation of a source-grounded n-channel MESFET in the power amplifier system of the invention, wherein FIG. 1 is a graph showing gate voltage (Vg) versus gate current (Ig) characteristics, FIG. 2 is a graph showing gate voltage (Vg) vs. drain current (Id) characteristics, and FIG. 3 is a graph showing drain voltage (Vds)xe2x88x92drain current (Ids) characteristics along with load curves.
In FIG. 1, a Vgxe2x88x92Ig curve 1 in case the Schottky barrier xcfx86B is significant indicates a MESFET to which the present invention is applied. On the other hand another Vgxe2x88x92Ig curve 2 in case the Schottky barrier xcfx86B stays less indicates a MESFET used as a comparison example through the inventors"" experimentation for evaluation. As is apparent from the definition of the Vf stated above, a voltage providing a current value Ig0 equivalent to a current of 100 xcexcA per gate width of 100 xcexcm is Vf1 in the case of the curve 1, and is Vf2 in the case of curve 2. Experimental investigation made by the inventors has suggested that Vf2 is approximately 0.6V whereas Vf1 is about 0.7V for example.
From such Vgxe2x88x92Ig characteristics, the Vgxe2x88x92Id characteristics are as shown in FIG. 2. A curve 3 designates the Vgxe2x88x92Id characteristic of the MESFET incorporating the principles of the invention, wherein Id begins to flow when Vg goes beyond Vth and wherein Id increases with an increase in Vg within a range up to Vmax1. And Id is saturated when exceeding Vmax1. Vmax1 is substantially equal to Vf1 as stated supra. On the other hand, the case of the MESFET of the comparative example is indicated by a curve 4, wherein although Id begins flowing when Vg exceeds Vth in the same way as in the previous case, Id becomes saturated when exceeding Vmax2. Vmax2 is nearly equal to Vf2 from the definition of Vf. The graph of FIG. 2 also shows input and output signals simultaneously. Letting a gate bias voltage be represented by V0, the maximum value of an input signal capable of being applied to the gate bias voltage as a gate voltage is Vmax1 in the case of application of the invention whereas the same is Vmax2 in the case of the comparative example. Accordingly the maximum value of the drain current acting as the intended output signal is Imax1 in the case of this invention whereas the same is Imax2 in the case of the comparative example. This may be summarized in a way such that the drain current of the invention capable of being extracted as an output increases from Imax2 up to Imax1 when compared to the comparative example.
This may be indicated by load curves as shown in FIG. 3. Specifically the MESFET of the present invention exhibits a load curve 5 which enables the gate voltage Vg to be applied up to about 0.7V thereby making it possible to drive the drain current up to Imax1; on the contrary, the MESFET of comparative example has a load curve 6 with a limitation in applying the gate voltage Vg merely up to about 0.6V, which in turn makes it impossible to drive the drain current of less than or equal to Imax2 at its upper limit. This investigation well demonstrates that use of the inventive teachings advantageously serves to increase a load current corresponding to a value of Imax1 minus Imax2 as compared to the comparative example while increasing the intended output accordingly.
Note here that although the consideration through experimentation made by the present inventors has revealed that the prior art MESFET measures about 0.6V in Vf as discussed previously, this value is less than the value of xcfx86B (for WSi, xcfx86B=0.75) as reported by the above-identified Japanese citation entitled xe2x80x9cCOMPOUND SEMICONDUCTORS,xe2x80x9d by Nikkan Kougyou Shinbun-Sha, Jan. 30, 1986, p. 165. The value Vf obtained by the inventors is observed to be less than such citation""s value in this way, and this fact is due to the following reasons. The citation""s value observation is such that electrode fabrication is done immediately after cleavage in vacuum environments in many cases, which encourages us to believe that the resulting value observed must be an xe2x80x9cidealxe2x80x9d value obtained in the state that surfaces are kept extremely cleaned. In contrast, the Vf value measured by the inventors is a value that was observed in real devices, which might accompany certain contamination on the surfaces thereof. In addition, with such real devices, thermal processing is done after gate electrode fabrication processes, resulting in creation of reaction between a metal and semiconductor, which metal constitutes the gate electrode. Further, with the real devices, the gate length stays generally shorter; if this is the case, the so-called gate edge effects can enter causing a leakage current to often take place due to such edge effects. These factors are overlapped one another letting Vf be finally observed at the value less than that as taught by the above-identified Japanese citation.
In addition, said MESFET may be any MESFET of the shallow depression type or the enhancement type. Such sallow depression or enhancement type MESFET is inherently designed to apply as its gate bias voltage a forward voltage of zero volts or low potentials, which will be an important technique for use when driving by a single power supply; even in such a case, the invention permits application of the gate voltage up to Vf1 (Vmax1) thereby enabling provision of a sufficient output, which in turn makes it possible to compensate for demerits in the single power supply-driven power amplifier system.
It should be noted that the channel region of said MESFET is made of a chosen compound semiconductor material of the direct transition type. In accordance with this power amplifier system, it is possible by utilizing high carrier mobility of the direct transition type compound semiconductor to make up the intended power supply amplifier system capable of operating at high speeds. In particular, an n-channel MESFET using electrons as its majority carriers is most effective to the trend of achieving higher speeds and thus may be applied to amplification of high frequency signals of 1 GHz or more.
Additionally the direct transition type compound semiconductor may typically include aluminum gallium arsenide (AlGaAs) or alternatively gallium arsenide (GaAs), by way of example.
(2) The power amplifier system of the present invention is such that in the above noted power amplifier system, a circuit for supplying the gate bias voltage comprises more than one ripple filtering capacitor.
Letting the gate bias voltage supplying circuit comprise the ripple filter capacitor makes it possible to improve the stability of the power amplifier system. More specifically, with the power amplifier system of this invention, it is possible when applying an input signal as superimposed with the gate bias voltage to derive a drain current as an output current; however, a power supply for supplying this drain current is a single unipolar power supply unit, which power supply is also operable to generate a gate bias voltage. Upon supplying a drain current Id from the power supply, a potential drop of rxc2x7Id can occur at a power supply terminal due to the presence of an internal resistivity xe2x80x9crxe2x80x9d of the power supply, thereby causing a gate bias voltage generator circuit also to suffer from appreciable influence of this potential drop. Especially with the power amplifier system of the invention, the potential drop rxc2x7Id""s influence becomes greater because of the fact that the applicable range of the gate voltage is expanded to increase a drain current capable of being taken out as the intended output. In view of this, with the invention, the gate bias voltage generator circuit is specifically designed to come with the ripple filter capacitor for preventing unwanted overlapping or mixture of high frequency noises into the gate bias voltage, thus increasing the stability of gate bias voltage. This makes it possible to well stabilize an operation of the power amplifier system. In addition, with the power amplifier system of this invention, requirement for the use of the unipolar power supply leads to a decrease in range capable of applying the gate voltage. Due to this, the resultant amplitude of an input voltage signal must be made smaller accordingly while at the same time causing the stability of the gate bias voltage relative to the input signal to become more severe relatively; thus, the effect of the ripple filter capacitor of the invention will become more significant.
Also note that the ripple filter capacitor is provided outside of a semiconductor substrate with the MESFET formed thereon. While the practical capacitance value of such ripple filter capacitor will be explained later in the description, such value is generally large; when an attempt is made to realize it by IC microfabrication technologies on the semiconductor substrate, the resulting formation area can become greater resulting in an increase in IC production costs. Hence, letting the ripple filter capacitor or capacitors be provided separately outside of the semiconductor substrate makes it possible to constitute the intended power amplifier system at low costs.
(3) The power amplifier system of the instant invention is such that in said power amplifier system, a layer made from an alloy of a metal constituting the gate electrode and a semiconductive material making up a channel region is formed at the interface between said MESFET""s gate electrode and channel region.
In accordance with this power amplifier system thus arranged, it is possible to achieve an improved MESFET structure with Vf of 0.65V or greater. With prior art MESFETs used in many cases, certain materials that hardly form any alloys in combination with semiconductor such as tungsten silicides have been chosen for use as the gate electrode to thereby attain thermal stabilities or alternatively utilizabilities of thermal processing steps during manufacturing procedures. On the contrary, with the present invention, it is proposed to force the material constituting the gate electrode and the semiconductor in the channel region to actively thermally react with each other thus forming an alloy layer at the interface therebetween. Forming such alloy layer in this way permits the intended Schottky junction to be formed between this alloy layer and the semiconductor in the channel region, thereby enabling reduction of any appreciable influence or interference of an interface energy level that exists at the interface between the semiconductor in channel region and the gate electrode metal. This in turn makes it possible to avoid occurrence of pinning effects thereby enabling formation of the Schottky barrier N in a way pursuant to the work function of such material. Whereby the Schottky barrier xcfx86B may be made greater to likewise increase Vf in value. In addition, as the alloy layer is formed in advance or xe2x80x9cpre-formed,xe2x80x9d the resulting thermal stability may also be increased. This in turn makes it possible to improve the operation reliability of the power amplifier system.
Another feature of the invention lies in that said alloy layer is formed at a level lower than the surface of the channel region. Forming the alloy layer at the specified level underlying the channel region surface in this way makes it possible to further reduce the influence of the interface energy level that can cause pinning effects.
A further feature of the invention is that said metal has its work function greater than the work function of tungsten silicides. As previously stated, the Schottky barrier xcfx86B between the alloy layer and the semiconductor is determinable depending on the work function of such alloy layer in the state that the pinning effects are suppressed. Due to this, while suppressing pinning effects, use of the scheme stated supra let the metal have its work function greater than that of tungsten suicides whereby the resultant Schottky barrier xcfx86B increases thus enabling achievement of the value of Vf which is greater than or equal to 0.65V. Practically, the metal may preferably be either platinum (Pt) or palladium (Pd). These metals per se are significant in work function; an alloy of these metals and a semiconductor material such as for example arsenic is also great in work function, which in turn allows formation of a significant Schottky barrier xcfx86B at the junction interface between platinum arsenide (PtAs) and GaAs.
Increasing the Schottky barrier xcfx86B in this way will also contribute to reduction of possible leakage currents between adjacent channels. This may result in a decrease in electrical power as consumed by the power amplifier system.
Additionally, although forming the alloy layer using either platinum or palladium may increase Vf in value in the way as previously stated, it has been affirmed through experimental investigation made by the present inventors that this Vf value is also variable depending upon semiconductive materials used for the channel region. More specifically, in the event that AlGaAs is employed as the semiconductor material while letting the gate electrode be made of platinum for example, Vf is at least 0.70V or greaterxe2x80x94typically, 0.75V or more or less. Alternatively in case GaAs is used as the semiconductor material while letting the gate electrode be made of platinum for instance, Vf is at least 0.65V or morexe2x80x94typically, falls within a range of from 0.67 to 0.73V. It may be considered that the Vf value behaves to vary between AlGaAs and GaAs are based on the presence of a difference in electron affinity therebetween. Additionally, Vf tends to exhibit distributivities in the range of 0.67 to 0.73V even where GaAs is equally employed due to differences in thickness of platinum. To be more specific, in the case of employing platinum as thin as 70 to 80 Angstroms (A), Vf ranges in value from 0.67 to 0.69V; in case platinum is as thick as 300 A then Vf ranges from 0.72 to 0.73V.
(4) The power amplifier system of this invention is such that in said power amplifier system, MESFETs involved are formed in a way separated from one another in units of semiconductor substrates while at the same time causing MESFETs and passive elements to be arranged separately from each other; or alternatively, MESFETs and more than one passive element making up amplifier circuitry are fabricated and integrated together on a single semiconductor substrate; or still alternatively, MESFETs and passive elements constituting such amplifier circuitry plus output matching circuitry operatively associated with the amplifier circuitry are all integrated together on a single semiconductor substrate.
In other words the power amplifier system of the invention is configurable in a discrete form while letting only part of amplifier circuitry be integrated into an IC chip (typically the one known as monolithic microwave IC or xe2x80x9cMMICxe2x80x9d) package or alternatively microfabricating it into such IC chip along with output matching circuitry operatively associated therewith.
(5) The power amplifier system of the invention is a mobile communications terminal device having a power amplifier circuit including a compound semiconductor MESFET for use as an active element for amplifying and outputting a high frequency signal, the MESFET having a source coupled to ground, a unipolar power supply for supplying the compound semiconductor MESFET with a drain bias voltage and a gate bias voltage, and an output matching circuit of the power amplifier circuit, featured in that the compound semiconductor MESFET permits, upon application of a forward DC gate voltage to a gate terminal with its source terminal grounded, the DC gate voltage to be greater than or equal in value to 0.65 V, the DC gate voltage causing a gate current value per gate width of 100 xcexcm to go beyond 100 xcexcA.
In addition, said gate bias voltage supply circuit may be arranged to comprise one or more ripple filtering capacitors that are provided outside of a semiconductor substrate with more than one MESFET formed thereon; further, said compound semiconductor MESFET includes a layer which is made from an alloy of a chosen metal and compound semiconductor and which is formed at the interface between the MESFET""s gate electrode and channel region made of compound semiconductor, the chosen metal including platinum (Pt) or palladium (Pd).
In accordance with such mobile communications terminal device thus arranged, a unipolar power supply unit may be used to successfully drive the power amplifier circuit while simultaneously enabling increase in output of the power amplifier circuit thus improving the performance of the mobile communications terminal device.
Technical advantages obtainable by some representative ones of the inventive teachings and features as disclosed above will be explained in brief summary as follows.
(1) It is possible to improve the gain of the high frequency power amplifier system inherently designed to operate with a unipolar power supply for driving the same.
(2) It is possible to reduce power consumption of the high frequency power amplifier system.
(3) It is possible to improve the stability of such high frequency power amplifier system.
(4) It is possible to increase the output of a mobile communications terminal device capable of being driven by a unipolar power supply at low power consumption levels, that is, capable of offering enhanced battery-use drivabilities for extended periods of time.